Don't know about him playing with Stamkos. Could happen. He could easily play with Filppula on the 2nd line too. That's two very solid two-way players and would give them a line that carries the heavy minutes defensively.
He will still continue seeing lots of PP whether he plays 1st or 2nd line. I think he can have a solid bounce back season and put 45-55 points. Big scale and I would play it safe and project him towards the lower end of it.
Callahan, like Brown, are currently undervalued after a poor season but I think they both still have several good years left in tank. In leagues with hits, they're still valuable contributors to fantasy teams.
That all said, I would not keep him over Schwartz or Letang. That's two highly skilled players who can put up lots of points on the board. Letang is an injury risk but if you own him, hold onto him. If he stays healthy, he will be huge player for your team. And Schwartz will be key player offensively for the Blues. Can't let go a young player like him that will only get better from now on. So while Callahan is a good own, you simply don't seem to have anyone on your team that he'd be upgrade over. His hits are nice but that area can be addressed later on while a high scoring D-men like Letang or young high scoring player like Schwartz are harder to come by.